I. Field of the Invention
The present invention relates to semiconductor lasers. More particularly, the present invention relates to a surface emitting laser assembly for high power, temperature stabilized wavelength operation.
II. Background Art
Conventional semiconductor lasers are configured for emitting light from an edge-cleaved facet of the device. Surface emitting lasers, which emit light from a top or bottom surface of the device, have an advantage over conventional edge emitting lasers in that the surface emitting lasers are readily adaptable for coupling to other optical components. The monolithic nature of the surface emitting laser structure also provides considerable advantages over hybrid devices in the way of mechanical and thermal stability. Surface emitting lasers also make possible large emitting areas which can be made to have narrow beam angles and high power outputs.
Grating coupled surface emitting lasers (GCSEL) have gratings etched into waveguides adjacent to the active region which effect feedback and coupling out of the laser emission. Grating coupled surface emitting lasers have an advantage over vertical cavity surface emitting lasers (VCSELs) in that fabrication of a multi-layer semiconductor mirror directly on the laser chip to achieve vertical light propagation (i.e., surface emission) creates a risk of defect centers in the laser due to the excessively thick growths necessary for high reflectance mirrors and often requires specially constructed electrodes.
Distributed Bragg reflector lasers are among the best wavelength stabilized diode lasers available today. According to the Bragg reflection principle, refractive index variations and thickness of the layers are chosen so that partial reflections interfere constructively at a narrow band of wavelengths but not at wavelengths outside the band. Light within the selected narrow band is coupled back into the laser active area enhancing single longitudinal mode operation, or multi-longitudinal mode operation with the modes restricted to lie within the bandwidth of the distributed Bragg reflector's reflection peak.
It is known that thermally-stabilized conventional DBR lasers may be constructed with layers of dielectrics possessing approximately equal and opposite changes in refractive index, see S. A. Gurevich, et al., Sov. Tech. Phys. Lett. 11(5), May, 1985 and Zh. I. Alferov, et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, Jun., 1987. In a commonly assigned co-pending application (Ser. No. 07/458,155), U.S. Pat. No. 5,043,991 the inventor discloses a method of stabilizing temperature-induced emission wavelength fluctuation in diode lasers by assembling a conventional Fabry-Perot laser together with a temperature-stabilized corrugated waveguide.
In another commonly assigned co-pending application (Ser. No. 07/579,933), U.S. Pat. No 5,056,099 the inventor describes a rugate filter for temperature stabilization of emission wavelengths of a conventional Fabry-Perot laser. The rugate filter comprises a dielectric film having a refractive index which varies in an approximately sinusoidal fashion throughout its thickness. The filter acts much like the Bragg grating described above. The dielectric film consists of a glass with a refractive index which does not vary with temperature by more than 10.sup.-6 /.degree.C.
In a third commonly assigned co-pending application (Ser. No. 07/579,597) the inventor discloses a rugate filter deposited on the second order grating of a grating coupled surface emitting laser for temperature stabilized laser emission at a desired wavelength. Such a device has the advantages of monolithic configuration but the disadvantages of exposure of the laser chip to additional steps in the fabrication process and considerable difficulty in creating lasers capable of operating at multiple center wavelengths on a single array.
It would be desirable to provide a filter for temperature stabilizing the emission wavelength of a grating coupled surface emitting laser which permits exploitation of the positive aspects of surface emitting lasers without exposure of the laser chip to additional processing., and which allows a single array of lasers to simultaneously emit light at multiple discrete wavelengths while maintaining stabilization of each wavelength. It is to this objective that the present invention is directed.